You are actually "tickling" the guinea pig. These sounds are not of pain, but possibly of mild annoyance based on where you are petting her. Often these types of sounds when petting guinea pigs are positive sounds because the guinea pig is "having a conversation with you." In this case, the guinea pig seems to be making slightly higher pitched sounds and is moving her head around (a sign of wishing this would stop). This is not hurting her, but she likely is uncomfortable (think if someone were ticking your thighs or armpits, it does not hurt, but gets irritating). Try petting her on her ears, head, chin, and back; if she makes similar sounds and limited head movement, she is having fun and you should stop petting her rear-end. If she makes no sounds or makes the same head movement, then this rear-end petting is okay.

Guinea pigs generally are sensitive on their rear-ends; they are ticklish there and something can sway side-to-side or roll over if tickled too much.